Probably sex, for me.
I’d just do it with someone who wasn’t who I originally did it with because she wasn’t great in bed. The entire moment itself was just embarrassing for the both of us, moreso for me because of the time it last for which wasn’t long and just her seemingly doing it to get it out of the way. There was no passion in it, it was just a matter of “oh lets get you not a virgin anymore” deal and I felt it was tainted.
That’s everyone’s first time, to a greater or lesser degree.
For me, there’s nothing I’d want to do for the first time again. Whatever it would be, I’m better at it now than I was then.